High reactance transformers



July 5, 1960 R. s. QUIMBY HIGH REACTANCE TRANSFORMERS Filed Oct. 28,1955 INPUT VOLTAGE INPUT VOLTAGE m wnn C Q 5 D m (cc cc L W M w fwxv KC1% m w 2 "W WWW E WW /QQO m y 5 A TTORNEV 0.! 2 .3 .45 .6 .81-0 TURNSRATIO A 2,944,208 3 I Patented 1 2,944,208 HIGH REACTANCE TRANSFORMERSRobert S. Quimby, Lexington, Mass, assignor to Raytheon Company, acorporation of Delaware Filed Oct. 28, 1955, Ser. No. 543,528

11 Claims. (Cl. 323- 18) This invention relates to transformers, and,more particularly, to a high reactance transformer having a controlwinding for obtaining a wide rangeof values of leakage reactance. i v lTransformers of the high reactance or leakage reactance type generallyconsist of a rectangular main core structure having one or more coremembers on which primary and secondary windings, or coils, are wound andone or more ferromagnetic shunts positioned adjacent the core members toprovide a shunt path for mag neticfiux traversing one of the coils. Asis generally known, a transformer of this type is capable of greatlylimiting thecurrent under short circuit conditions, the value of thecurrent so limited depending upon the length of an air gap or gapsbetween the shunt and the adjacent portion or portions of the main core.Control of the leakage reactance and, in like manner, control of theshort circuit flux path is obtained by an adjustment of -the length ofthe air gap which prov-ides a sensitive shunt for flux linking thecoils, which, with the usual manufacturing techniques, generallyrequires careful adjustment to provide a leakage deactance of propervalue to meet predetermined tansformer operating require ments. Inaddition, it becomes increasingly diflicult, with conventionalproduction line techniques, to maintain close tolerances duringpunchingvand assembly of the laminations making up the main core andshunt iron assembly for a plurality of transformers and, consequently,it becomes difficult to maintain substantially constant the desiredlength of the air gap. Ina majority of cases, therefore, individualadjustment of the air gap between the shunt iron-and the main corebecomes necessary, resulting in the expenditure of additional time andeffort as well as in extra labor andtesting costs. It is, therefore, anobject of the invention to provide a novel leakage reactance-typetransformer, in which it is possible to affect precisely predeterminedvalues of leakage reactance without the use of avaiiable air gap, and ina manner which can readily be established by design. An advantage ofapparatus embodying this invention is that it may be used as acurrent-limiting transformer in which accurate control of leakagereactance is achieved in a simple and convenient,manner. Thiscurrent-limitingefi ect maybe used, for example, in connection withlimiting the initial heating of the filament of certain types of hot.cathode tubes wherethe filament, due to its initial low reistance whencold, .might suffer shock damage from the application of heavy filamenrtcurrent from astandard-type transformer. 1 I

. In accordance with this invention, accurate control of transformerleakage over a wide range of values is obtained without the use of anair'gap shunt through the use of a three-coil transformer comprising amagnetic core arranged to be operated at a relatively moderate fluxdensity below the knee of the B+-H curvev of the core material, the corecomprising three'co'rel members on which arewound a primary winding, asecondary orload winding, and a novel control winding connected inseries with the load winding. The primary winding is wound on the centercore member in order to provide separate flux paths linking the twoouter legs. When the control winding on one outer leglis connected inseries with the load winding, the distribution of the amount of fluxsupplied by the primary winding linking the two outer windings,

' and, consequently, the leakage reactance, can be controlled bychanging the reluctance of the leak-age flux path. It has been foundthat this is easily achieved by changing the ratio of turns on thecontrol winding with respect to the turns on the load winding, the totalnumber of turns on both windings, and the open circuit transformeroutput voltage, remaining the same. In this manner, the ratio of turnson the control leg with respect to the number of turns on the load legcontrols the short circuit current flowing through each leg. In anotherembodiment of the invention,.the number of turns on the load and controlwindings, and, in turn, the degree of leakage flux traversing thecontrol flux path is varied by providing each winding with a series oftaps, so that as the number of turns on the control winding isincreased, the number of turns on the load winding is decreased in orderto maintain the total number of turns on the two windings, and thus thetransformer output voltage, at all times constant. In like manner, thecontrol of degree of leakage and the value of the short circuit currentwhich can be obtained thereby, is simplified to an unexpected degree.

Other objects and a better understanding of the invention may be had byreferring to the following specification and claims in connection withthe accompanying drawings in which:

Fig. 1 is a diagrammatic view of a leakage reactance transformer shownwith a winding on the leakage flux P Fig. 2 is a diagrammatic view of amodification of Fig. 1 showing taps on the windings on the leakage fluxpath and on the load flux path; and

Fig. 3 illustrates the changes accomplished in the reactance of thetransformer by application of the invention thereto.

Referring now to Fig. 1, there is shown a magnetic core having two outerflux paths, the one including the core member iii, the other includingthe core member 12, with the member 11 being common to both flux paths]The flux paths including the members 10 and 12 is a closed path whichhas a cross-sectional area large enough to permit the magnetic core tobe operated at a relatively moderate flux density below the knee of theBH curve of the core material in a manner that will be described indetail below. The two flux paths are magnetized by a primary winding 13on the center member 11, winding 13 receiving its energy from a singlephase alternating current source 9. As is well known, the sum of thethrough outer core members 10 and 12 is substantially equal to the fluxin center member 11. Located on outer core members 10 and 12 arewindings or coils 14 and 16 which, in this embodiment, are connected inseries with one another to form asecondary or output circuit connectedin series with load 15. Thus, flux caused by current flow in the primarywinding 13 passes through the center member 11, and then divides andflows through the outer members, thereby to produce a reactive effect oroutput voltage in the secondary circuit. It should be noted, however,that coil 14 is designated as the load winding and coil 16 as thecontrol winding, respectively, and that, because these windingsareconnected in series to form the secondary circuit, they can beinterchanged as long as theturn's ratio of the secondary circuit ispreserved. Furthermore,

if load winding 14 and control winding 16 were opencircuited, ordisconnected from load 15, as shown in Fig. 2, the flux produced by theprimary winding divides equally between the two outer legs, and thevoltage per turn of each outer winding becomes one-half the voltage perturn produced by primary winding 13. It will also be apparent that themanner in which the flux in center member 11 divides between the main orload core member 19 and the shunt or leakage path 12 depends upon therelative reluctances of these two paths, and that by increasing ordecreasing the reluctance of the leakage path, the proportion of fluxtraversing the load magnetic flux path may be increased or decreased. Inlike manner, the leakage reactance of the transformer and, in turn, itscurrent-limiting effect may be controlled, in a simple and reliablemanner, Without the use of a magnetic shunt and attendant air gap bycontrolling the reluctance of the leakage or control magnetic flux pathwhich, in accordance with the invention, has been found to besubstantially proportional to the ratio of turns between the controlwinding 16 and the load winding 14. It should be noted, however, loadwinding 14 and control winding 16 may be connnected either into seriesopposition to produce diiferent reactive effects or, as here, in seriesaddition, providing the circuit is fed from a common flux source, suchas center member 11. However, an important advantage of this inventionis due to the fact that the control Winding 16 is connected in seriesaddition with the load winding 14, so that each turn contributes to thetotal output voltage, making unnecessary the additional expenditure ofcopper for biasing or control windings not directly contributing to theoutput voltage of the secondary circuit.

Referring now to Fig. 2, there is shown a modification of thearrangement of Fig. l, with windings having additional turns on outercore members 10 and 12 and on center member 11. Thus, a load winding 20is connected in series with load i and control winding 22 to form theusual secondary and load circuit for primary winding 21, which isconnected to the single-phase alternating current source 9. However, toobtain different values or transformer leakage, load Winding 20 isprovided with taps 39 to 34- inclusivc, and control winding 22 with taps49 to 43 inclusive, which, in turn, are connected to apropriate contacts50 to 54 inclusive and 60 to 64 inclusive on rotary switch 24, in themanner shown. In addition, taps 49 and 65 provide a means fordisconnecting control winding 22 from load winding 2{), when it isdesirable to use the transformer in a conventional manner. 25 and 26and, as shown, is positioned to shunt out control winding 22 from theload circuit. In this manner, the turns ratio between the load andcontrol windings, and hence the reactance, is adjusted as rotatableswitch 24 is progressively advanced to link each set of the remainingcontacts connected to the taps on the load and control windings. Asnoted, if all turns on the leakage flux path 12 are disconnected bymeans of switch 24 remaining in its initial position, as shown, maximumtransformer leakage is obtained, and the leakage flux traverses controlmember 12 unimpeded by reluctance which is set up when coil 22 isenergized. Thus, when all the secondary turns are on core member 10,core member 12 acts as a conventional magnetic shunt, and when currentis drawn from secondary winding 20 by load 15, fiux is forced from coremember to core member 12, and the voltage across the secondary windingdrops to a lower value in accordance with well-known transformer theory.Also, transformer operation is equivalent to that of a conventionaltransformer having a shunt provided with a very small air gap. Becausethe amount of flux which can be shifted from core member 10 to coremember 12 is related to the ratio of turns on the respective members,the use of one winding only, representing a turns ratio Switch 24 isprovided with rotatable contacts pedance.

of zero, should theoretically produce a zero flux in load member 10 whenthe winding is short-circuited, but, due to the iron losses in the corematerial, some flux remains in load member 10 and, thus, the use of thissingle winding provides a high but finite impedance. As noted, if equalturns on each outer core member are connected into the secondarycircuit, minimum transformer leakage is obtained. By proportiom'ng theturns between the two outer legs, the degree of leakage and,consequently, the short-circuit current can be controlled, and, since ithas been found that the leakage is proportional to the turns ratio,transformer reactance can be accurately controlled without the use of anair gap.

Referring again to Fig. 2, when switch 24 is rotated in a clockwisedirection to engage contact 50, connected to tap 30 on load winding 20,it also engages contact 64, connected to tap 43 on control winding 22.In this position a single turn is subtracted from the load winding andadded to the control winding in the manner shown. The control winding isnow placed in series with the load circuit 15, and leakage flux, whichtraverses control member 12, is now impeded by the reluctance set up bythe first turn of the control winding. As switch 24 is progressivelyadvanced through contacts 51 to 54 connected to corresponding taps 31 to34 on load winding 20, the number of turns on the load windingdecreases, as shown, while the number of turns on the control windingincreases by the action of contact 26 advancing through contacts 64 to60 connected to corresponding taps 43 through 40 inclusive. Thus, aseach successive turn is added to the control winding, the reactance ofthe transformer decreases until an equal number of turns on each windingis reached, corresponding to switch positions 54 and 60. In thisposition, the use of an equal number of turns on each leg, representinga turns ratio of one, results in a low but finite transformer impedancedue to iron losses, rather than a theoretically zero im- Thus, inpractical applications, a transformer having a single control windingcan be designated as having a percent reactance, while the lowesttransformer reactance obtained with equal coils on both legs will besubstantially 10 percent of this value. However, the desired transformerreactance will usually be obtained from turns ratios from between 0.1and 0.8, as will be described with reference to Fig. 3, where it hasbeen found that a substantially linear relationship exists andtransformer reactance may be accurately calculated in a manner that willbe described in detail below.

Of course, it is to beunderstood that while switch 24 may be rotated toposition 54 and 60 to provide an equal number of turns on each winding,it is possible that additional turns could be added to the controlwinding which would have the efiect of making the control windingoperate as a load winding, and the present load winding then, in effect,become a control winding, the reactance of the transformer remainingproportional to the turns ratio of the aforementioned windings. For thisreason, this embodiment shows the control winding having no more thanthe same number of turns as the load winding.

By utilizing the principles of the invention, as described above, atypical leakage reactance transformer. as shown in Fig. 1, can beconstructed using standard E- and I-shaped laminations interleaved byalternately reversing the position thereof to provide a minimum air gapas is customary in conventional transformer designs. Thus, as shown inFig. 1, this typical leakage reactance transformer consists of a closedcore having a transverse over-all dimension of three inches, the lengthof the three core members contained therein being 2.5 inches over-all,and alternately interleaved with E and I laminations in a conventionalmanner. The width of the two windows is approximately one-half of aninch, as is the width of the material making up the main and controlmagnetic flux paths. In order that the transformer will the primary orcenter member is at least as wide as the outer members. Furthermore, thecore material consists of 26-gauge, 4 percent silicon steel, grade 72,in the form of sheets which have been punchedjinto E and I laminations.It is to be understood, however, that other methodsof interleaving maybe employed or other methods of core construction, such as wound cores,may be used. In addition, the primary coil in the present embodimentconsists of approximately 2500 turns of No. 29 wire, which is woundaround the center leg in the direction shown in Fig. 1. The load coilcomprises a thousand turns of No. 29 Wire, the control winding having500 turns of the same size wire wound in the direction indicated. Thenumber of control turns may be calculated in the manner described indetail below.

For purposes of explanation, Fig. 3 shows curve 80 representingpercentages of total transformer reactance or impedance recorded fortypical turns ratios generally having values between 0.1 and 0.8. Thelimits of the percentages of transformer impedance shown at 81 and 82may be obtained by measuring the open circuit voltage and the shortcircuit current and,'as is well known, dividing the former by the latterto obtainthe' maximum transformer reactance for this particular design.Thus, using the curve shown in Fig. 3, when the desired values of theopen circuit voltage and short circuit current are given, these twofactors can be used to establish the number of turns on the load windingand thus the design of the novel transformer, shown in Fig. 1, inaccordance with the invention. For example, assuming the presenttransformer requirements are for an open circuit voltage of 35 volts,and a short-circuit current of 0.230 amperes, the transformer impedanceis equal to 35 divided by 0.230, or 152 ohms required impedance. It isdetermined from well-known calculation that one thousand turns of thecorrect size wire, No. 29, will fill the available space in the windowof the load coil 14 and that this number of turns will produce a loadcoil impedance of 1000 ohms. Referring now to the'curve shown in Fig. 3,the impedance of 1000 ohms is related to the curve as '100 percentimpedance at a turns ratio of zero. This is shown on the curve 80 at 81representing substantially no control turns. As noted, 82 corresponds toa turns ratio of one, or substantially equal control turns and loadturns. Now, the required impedance is approximately 150 ohms, which is15 percent of the maximum impedance and 83 on the curve 80 shows that 15percent impedance is' equal to a turns ratio of 0.5. Since thisjratio isthe ratio of control turns to load turns on the secondary winding, thenumber of control turns. is equal to 1000 multiplied by 0.5, which isequalto 500. Thus, the secondary winding of the transformer shown inFig. 1 has 1000 turns on the load coil and 500 turns on the controlcoil; Since the sum of thevoltages for the two coils must equal 35volts, the volts perturn in each coil must equal 35 divided by 1500turns,which is; equal to 0.0233. Then, according to well-knowntransformer theory, since the volts per turn on the center winding mustbe twice this value, the primary coil will have a volts-per-turn valueof 0.0466. Now in order to operate from the usual 115-volt alternatingcurrent line, the number of turns required will, therefore, be 115divided by .0466 which is equal to 2460 turns of wire, which issubstantially equal to 2500 turns, as mentioned previously. Of course,the number 29 wire size may be established in the well-known manner fromthe total secondary volt amperes. In this manner, therefore, it ispossible to transfer the burden of accurate control of transformerreactance from the assembly line to engineering design by theelimination of the air gap shunt with its attendant dilficulties.

Although the invention has been described with a certain degree ofparticularity, it is understood that it is not to be limited to theparticular embodiment described herein example, the output power of theload 'w'ind ing can becontrolled by varying the value of a resistoracross the control winding, or similar control maybe obtained bysupplying current from a separate sourceto the control winding, therebypermitting control of very high voltages or high currents generallybeyond-the limitsof conventional controls, such as rheostats connectedin series with such load circuits. Furthermore, while the load in theabove-described embodiment, shown in Fig. 2, is connected betweenthe-load and control windings in the manner shown, it also may be placedat any other point within the series secondary circuit. Also, theloadwinding may consist of separate symmetrical windings connectedtogether to provide a center tap which may be grounded to meet externalcircuit requirements.

In addition, it will be appreciated that many variations of the featuresshown and described herein in connection with the single embodiment ofthe 'invention illustrated will occur to those skilled in the art towhichthe invention relates. 'It is, therefore, intended that the claimswhich'follow-shall not be limited by the particular details of theillustrated embodiment but rather by the prior art. I

What is claimed is: i

1. A reactance transformer comprising in combination a closed magneticcore having three coremembers, a primary winding only on one core memberadapted to be connected to a source of alternating current and toprovide a source of-fiux equally traversing said other core members, asecondary circuit adapted to be connected to a load comprising a Windingon the first of said other core members connected in series additionwith a winding on the second of said other core members, said lattercore winding co-acting with said first core winding to determine thepercentage of leakage flux applied to said latter core winding from saidfirst core winding in response to said load connected to said secondarycircuit.

2. A reactance transformer comprising in combination a closed magneticcore having three core members, a primary winding on only one of saidcore members adapted to be connected to a source of alternating currentand to provide a source of flux traversing said other core members, anda secondary circuit comprising a winding on the first of said othervcore members connected in series addition with a winding on a second ofsaid other core members, said second core winding co-acting with saidfirst core winding to provide. a flux leakage path for flux normallytraversing said first core member, said leak ageflux being responsive tothe ratio of turns of said first and second core members.

3. A variable reactance transformer comprising in combination a closedmagnetic core having a load ma-gmeans for supplying said primary windingwith. alter: C

natin-g current, a secondary circuit comprising a load winding linkingsaid loadrnagnetic flux path and a con-.

trol winding linking said control magnetic flux path, said windingsconnected in series addition and in such a manner that the percentage ofleakage flux in said control magnetic flux path changes in response to achange in the number of turns of said control and load windings linkingsaid control and load paths, said total number of turns of saidsecondary circuit remaining constant.

4. A' leakage reactance-type transformer comprising in combination,first, second and third nonsaturable magnetic flux paths, a primarycircuit comprising a winding of a predetermined number of turns linking:only said second flux path and adapted to be energized by a source ofalternating current to produce oppositely flowing flux in said first andthird flux paths, a secondary circuit comprising windings linking saidfirst and third flux paths connected in series with a load, the amountof flux linked by said third winding with respect to said first windingbeing proportional to the ratio of turns on said first and thirdwindings.

5. In a leakage reactance-type transformer comprising a closed magneticcore of a predetermined value of leakage reactance having three coremembers and a nonsaturating winding of a predetermined number of turnson each member, means for supplying only one of said windings linkingonly one member with a source of alternating current, means connectingsaid windings on the two other core members in series addition withrespect to an output load circuit, and switching means for chang ing thenumber of turns on one member with respect to the number of turns on theother member while maintaining constant the total number of turns ofsaid two members, thereby changing the leakage reactance of saidtransformer from said predetermined value in response to a change inload in said output circuit.

6. A leakage rcactance-type transformer comprising a closed magneticcore having first, second and third core members, and a nonsaturatingwinding of a predetermined number of turns on each member, said windingon said second core member in a circuit separate from said otherwindings and adapted to be connected to a source of alternating current,said third core member providing a leakage flux path adapted to shuntpart of the fiux of said first core member around said third coremember, said third winding connected in series addition to said firstwinding and cooperating with said first winding to control thepercentage of said flux from shifting said first core member throughsaid third core member, thereby changing the leakage rcactance of saidtransformer in proportion to the ratio of turns of said first and thirdwindings.

7. A leakage reactance transformer comprising a mag netic core havingfirst, second, and third nonsaturable core members and windings of apredetermined number of turns on each core member, said first and thirdwindings on said first and third core members being connected in seriesaddition relationship with respect to an output load circuit, saidsecond winding connected in a circuit separate from said other windingsand adapted to be energized by a source of alternating current, saidthird core member having a predetermined number of turns to produce achange in leakage reactance in said transformer in response to a changein load in said output circuit, said leakage reactance beingproportional to the ratio of turns of said first and third windings.

' 8. A reactance transformer comprising a closed magetic core havingthree core members providing three nonsaturable flux paths, means actingin connection with only one core member to provide a source of magneticflux equally traversing said other core members in the absence of aload, means on said third member co-acting with means on said firstmember to control the proportion of flux distributed from said sourcethrough said first. and third core members in response to a load,thereby to control the reactance of said transformer, said means on saidthird core member coupled in series with said means on said first coremember to provide anoutput current varying in response to a change inload applied to saidtransformer.

9. A nonsaturable high reactance transformer system adapted to operatewithout an air gap magnetic shunt for leakage flux, a plurality ofmagnetic core members having continuous magnetic core flux pathsintercom nected at a common junction, an input power source connected toa primary winding linking only one of said members, first and secondwindings on at least two of said core members, said first and secondwindings connected in series aiding with a load circuit and coactingwith said load circuit to control the percentage of leakage fiux flowingthrough one of said core members other than the member to which saidprimary winding is connected.

10. A leakage reactance transformer comprising a closed magnetic corehaving three core members adapted to provide three nonsaturable fluxpaths, means coacting with one of said core members to provide a sourceof magnetic flux traversing and other core members,

first winding means for setting up a counter flux in re-' sponse to aload connected to said first winding on a first of said other coremembers to oppose said flux traversing said first core member, saidother core member providing a leakage flux path, thereby to attain arelativcly high leakage reactance, and, second winding means on saidother core members adapted to control the percentage of leakage fluxapplied to said other member from said first core member, said secondwinding means connected in circuit with said first winding means andsaid load.

11. A leakage reactance transformer comprising a closed magnetic corehaving first, second, and third nonsaturable core members providingthree nonsaturable flux paths, a first winding on only the first of saidcore members adapted to be energized by a source of alternating currentto provide a source of flux at all times equally traversing said secondand third core members, said third core member arranged to act as aleakage flux path for a portion of said flux traversing said second coremember, and a secondary circuit comprising windings on said second andthird core members adapted to be connected to a load circuit, said loadcircuit cooperating with said leakage flux path to control thedistribution of flux in said second and third core members, said thirdwinding connected to said second winding in a manner to oppose said flowof leakage flux through said third core member when said secondarycircuit is connected to said load circuit.

References Cited in the file of this patent UNITED STATES PATENTS1,109,244, Murphy Sept. 1, 1914 2,186,207. Rampachcr Jan. 9, 19402,440,540 Farr Apr. 27, ,1948 2,440,984 Summers May 4, 1948 UNITEDSTATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. 2,944,208 m5, 1960 Robert S. Quimby It is hereby certified'that error appears inthe above numbered patent requiring" correction and that the saidLetters Patent should read as corrected below.

Column 1, line 36 for deactance" read reactance column 4, line 55,before "then" insert would column 7, line 31, strike out "from" andinsert the same after "shifting" in line 32, same column 7; column 8,line 23, for -"and" read said Signed and sealed this 1st day of August1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of PatentsUNITED STATES PATENT OFFICE CERTIFICATION OF coRREcTIoN Patent Nou2,944,208 July 5, 1960 Robert S, Quimby It is hereby certified'thaterror appears in the above numbered patent requiring" correction andthat the said Letters Patent should read as corrected below.

Column 1, line 36 for "deactance" read reactance column 4 line 55 before"then insert would column 7 line 31 strike out "from" and insert thesame after "shifting" in line 32, same collumn 7; column 8, line 23, for"and" read said Signed and sealed this 1st day of August 1961,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

